This invention relates to drive systems for military vehicles, particularly wheeled vehicles using limited slip differentials and run-flat tires.
Military forces have sought to improve the mobility of wheeled vehicles by substituting limited slip differentials for conventional differentials. A differential is a gear system that distributes driving torque to both right and left wheel axles as it compensates for the different distances that the two powered road-wheels must travel when the associated vehicle is turning in one direction or another. For example, when the vehicle is turning to the left, the rightmost powered wheel must travel farther (faster) than the leftmost powered wheel in order to avoid wheel skid and resultant tire wear. The differential divides the power flow to the two axles while permitting one wheel to rotate faster than the other.
When a conventional differential is used, the tractive engagements between the tires and the terrain produce reaction forces that cause orbiting gears in the differential to absorb lost motions associated with vehicle turning maneuvers.
The conventional differential is not completely effective when one of the powered vehicle wheels is operating on slippery terrain, e.g. ice, mud, soft sand, etc.; power delivered to the slippage prone wheel causes it to spin instead of delivering a reaction force back to the orbiting gears in the differential.
Limited slip differentials have been developed to avoid wheel-spin problems associated with conventional differentials. The limited slip differential includes a slippable clutch in the gear system for inducing an artificial frictional load on at least one of the axles, sufficient to prevent the undesired wheel spin. The clutch causes the two aligned axles to rotate together, with limited slippage therebetween sufficient to compensate for differences in road wheel speed associated with vehicle turning maneuvers. With a limited slip differential each powered axle receives some power, rather than having all of the power diverted into the slippage-prone wheel. The limited slip differential allows the associated vehicle to turn corners, while at the same time offering improved tire-terrain traction when the vehicle is traversing slippery terrain.
Vehicle mobility is enhanced by using the limited-slip differential in combination with run-flat tires; such tires give the vehicle a "get home" capability after a tire blow-out (failure); however, excessive axle traction can cause rapid heating of tires running flat under certain conditions. Military strategists are concerned with the effect of tire blow-outs, particularly when the vehicle is moving in enemy terrain or when subjected to enemy fire. A tire blow-out (due to a sharp object or enemy fire) can quickly immobilize the vehicle, leaving the vehicle and soldiers defenseless.
When a conventional tire on a powered wheel blows out it quickly assumes a depressurized flat condition, while the wheel continues to rotate; the depressurized tire undergoes a severe abrading action due to the speed differential between the rotating wheel and the stationary terrain surface. After a comparatively few revolutions the tire carcass assumes a twisted broken condition only partially engaged with the wheel rims; the relatively thin wheel rims and tire carcass begin to dig into the terrain, immobilizing the vehicle. The immobilized vehicle is ineffective for continued military operations.
To at least partially overcome the disadvantages of conventional tires there have been developed various run-flat tire constructions. Typically a run-flat tire includes a miniature insert structure of annular configuration. The annular insert structure is located within the tire carcass in surrounding relation to the wheel. The insert occupies the axial space between the wheel rim and tire carcass.
In some cases the annular insert structure is free to rotate relative to the wheel rim. In other cases the annular insert structure is bolted or otherwise affixed to the wheel. In either case the annular insert structure provides a relatively wide outer peripheral surface designed to engage the inner face of the tire carcass (in line with the tire tread area) to keep the wheel-tire assembly operating. The aim is to avoid the rim dig-in problem previously described.
When the military vehicle is using limited-slip differentials in combination with run-flat tires, any powered axle will continue to receive rotary power after the associated tire has been depressurized (by enemy fire or otherwise). Continued application of torque from the limited slip differential to the depressurized tire can produce early tire destruction and possible compromise of the tire insert action. Tractive forces between the terrain and the tire tread area cause the wheel flanges to spin on the tire carcass, with tire destruction consequences. The problem is caused at least partly by the fact that a limited-slip differential is used for supplying motive power to the powered axles.
The present invention seeks to at least partially overcome problems associated with the combinational usage of limited-slip differentials and run-flat tires. Applicant substitutes a driver-controlled "modified" conventional differential for the limited slip differential, while continuing to use the run flat tires. The conventional differential is modified in the sense that a manual locking means is substituted for the slip clutch used in limited-slip differentials.
The proposed differential has two operating modes. In an "unlocked" mode the differential functions like a conventional differential; the unlocked mode is used during normal operations on dry non-slippery terrain (rough terrain or smooth pavement). In a second "locked" mode the differential operates as a solid connection between the two axle sections; gearing within the differential carrier connects the axle sections so that each axle section rotates at the same rate. The second "locked" mode is used only when the vehicle is to be operated on slippery terrain, e.g. ice, snow, mud, and sand, where the unlocked differential would not supply sufficient tractive effort for forward motion.
The "unlocked" mode will be used during most of the time, especially when the vehicle is operating on tractive terrain at highway speeds, e.g., above twenty five miles per hour. The "unlocked" mode will also be used when one or more tires are punctured, e.g. after being struck by enemy fire. A flat tire has an effective diameter that is about thirty percent less than that of an inflated tire; therefore the flattened tire must rotate farther (faster) than the other tires in order to keep the vehicle running in a desired direction.
In its "unlocked" mode the differential will compensate for variations in tire rotational speed associated with a tire blow out. Most of the propeller shaft power will be directed to the inflated tire, leaving the blown tire to essentially free wheel on the terrain. This is believed to be a better run-flat action than that produced with a limited slip differential, wherein excessive power tends to be supplied to the slip-prone axle section (associated with the flat tire); the flat tire is then subject to early tire destruction.
The present invention relates to a wheeled vehicle using run-flat tires in combination with differentials that are adjustable between locked and unlocked operating modes. The invention seeks to provide the "slippery terrain" advantages of the limited slip differential and the "tire life enhancement" advantage of the conventional differential.
The differential used in this invention is in some respects similar to a differential shown in U.S. Pat. No. 4,526,063 issued to T. L. Oster on July 2, 1985. My invention relates primarily to an improved mechanism for remotely operating the differential adjustment means between the locked and unlocked modes.
I propose an operating mechanism that includes an air cylinder means and manually-operable valve for alternately supplying pressurized air to the air cylinder means or withdrawing pressurized air from the cylinder means. The cylinder means operates a clutch that effectively adjusts the differential between its locked and unlocked operating modes.
The air cylinder means can be located directly on the differential carrier in close physical proximity to the aforementioned clutch. The valve can be located in the vehicle cab or at some other point remote from the differential. When the valve is located in the vehicle cab the valve can be equipped with a handle operator for direct manual actuation by the driver. If the valve is located at some other point in the vehicle the valve can be solenoid-operated; a manual electric switch on the dashboard can then be used to operate the valve. As an alternate configuration, the air cylinder operation may be modulated by a pressure-control valve at the driver's station to apply a controlled amount of slippage from zero to 100% lock-up through conventional clutch packs between the orbiting differential gears and one drive axle. In this configuration, driver-selected variable slip control would provide optimum mobility of the run-flat tires, with the least tire wear friction.
The invention contemplates an improved mechanism for operating a vehicle differential between an unlocked mode and a locked mode.